Information recording media for industrial use and consumer use have been broadly used as a result of advance in research for optical information recording technologies in recent years. In particular, optical information recording media such as CDs and DVDs capable of recording information with greater density become popular. Such an optical information recording medium includes a transparent substrate formed with an information surface having a concavo-convex shape such as pits representing information signals and guide grooves for tracking recording/reply light, an information layer (for example, laminated metal film or laminated thermally recordable thin film material) formed on the transparent substrate, and a protective layer (for example, resin layer or transparent substrate) for protecting the information layer from, for example, moisture in the atmosphere. Information is replayed by detection of changes in an amount of reflected light of a laser beam irradiated on the information layer.
For example, in the case of CDs, a resin substrate of approximately 1.1 mm in thickness is prepared. A concavo-convex information surface is formed on one surface of the resin substrate. A metal thin film or a thin film material is laminated on the resin substrate to form an information layer. Subsequently, radiation curable resin such as ultraviolet curable resin is coated to form a protective layer, so that a CD is thereby produced. Information signals are replayed by a laser beam which enters from the substrate side, and not the protective layer side.
In the case of DVDs, a resin substrate of approximately 0.6 mm in thickness is prepared. A concavo-convex information surface is formed on the resin substrate. A metal thin film or a thin film material is laminated on the resin substrate to form an information layer. Subsequently, a separately prepared resin substrate of approximately 0.6 mm in thickness is affixed by using resin such as ultraviolet curable resin, so that a DVD is thereby produced.
There are more demands for an increase in capacity of the foregoing optical information recording media. In order to meet the foregoing demands, DVDs with multi-information layers have been proposed. Such a DVD comprises a dual information layer, and an intermediate layer of approximately several ten μm in thickness formed between the information layers.
In addition, there are more needs for next-generation optical information recording media with greater density and higher capacity than DVDs according to diffusion of digital high definition broadcasts in recent years. High-capacity media such as Blu-ray discs have been proposed as such a next-generation of optical information recording media. Such a high-capacity medium includes a substrate of 1.1 mm in thickness with a concavo-convex information surface, an information layer formed by laminating, for example, a metal thin film on the information surface of the substrate, and a protective layer of approximately 0.1 mm in thickness formed on the information layer. A Blu-ray disc has a narrower track pitch of the information and smaller pits than a DVD. Thus, a spot of a laser beam for recording/replaying information needs to be sharply narrowed on the information layer. A shorter wavelength of blue-violet laser beam (wavelength: 405 nm) is used for replaying a Blu-ray disc. An optical head comprising an objective lens with a numerical aperture (NA) of 0.85 is also used to narrow down the spot of the laser beam on the information layer. If the size of the spot decreases, however, it becomes more sensitive to tilt of the disc. Even slight tilt of the disc causes an aberration of the beam spot. The aberration of the beam spot results in strain in the narrowed beam, so that there are problems such as poor record/reply. In order to overcome the foregoing drawbacks, in the case of a Blu-ray disc, thickness of the protective layer, which is disposed on the laser incident side of the disc, is set to be extremely thin (approximately 0.1 mm).
Like DVDs, an increase in memory capacity has been also proposed for the next-generation of information recording media with high capacity such as a Blu-ray disc.
FIG. 13 is a schematic cross sectional view of a dual-layered Blu-ray disc including two information layers. A dual-layered Blu-ray disc is now described with reference to FIG. 13.
A molded resin substrate 201 is prepared. A concavo-convex first information surface 202 is formed on one surface of the molded resin substrate 201. A metal thin film or a thermally recordable thin film is laminated on the first information surface 202 to form a first information layer 203. A resin layer 204, which is substantially transparent to recording/reply light, is formed on the first information layer 203. A concavo-convex second information surface 205 is formed on the resin layer 204. A metal thin film which is translucent to recording/reply light or a thermally recordable thin film material is laminated on the second information surface 205 to form a second information layer 206. Resin, which is substantially transparent to recording/reply light, is coated so as to cover the second information layer 206 to form a protective layer 207. It should be noted that the term “substantially transparent” as used herein means that the layer has transmittance approximately at 90% or greater for the recording/reply light. In addition, the term “translucent” as used herein means that the layer has transmittance from 10% to 90% for the recording/reply light. A laser beam enters from the side of the protective layer 207 of the dual-layered Blu-ray disc. The focal point of the laser beam is focused on one information layer of the first and second information layers 203, 206, which works for record/reply, so that signals are recorded and/or replayed. It should be noted that the molded resin substrate 201 is approximately 1.1 mm in thickness. In addition, the resin intermediate layer (resin layer 204) is set to approximately 25 μm in thickness. The protective layer 207 is set to approximately 75 μm in thickness.
FIGS. 14A to 14F schematically show a process of producing a stamper which is used as a metal mold for producing the molded resin substrate 201 of the information recording medium. A standard manufacturing method of the foregoing dual-layered Blu-ray disc is now described with reference to FIGS. 13 and 14. It should be noted that principles of the manufacturing method of the dual-layered Blu-ray disc described below may be applied to a multi-layered Blu-ray disc comprising three or more information layers.
As shown in FIG. 14A, a photosensitive material such as a photoresist is applied on a master disk 301 made from a material such as a glass disk or silicon wafer to form a photosensitive film 302. Subsequently, an exposure beam 303 such as a laser beam or electron beam is irradiated on the photosensitive film 302 to perform exposure of patterns such as pits and guide grooves. Meanwhile, character information in the inner circumferential part with an aggregate of pits or grooves is exposed for identification of the master disk 301.
In FIG. 14B, hatched areas in the photosensitive film 302 are the exposed portions 304 which have been exposed with the exposure beam 303. A latent image is formed from the exposed portion 304 by the irradiation of the exposure beam 303 to the photosensitive film 302 described with reference to FIG. 14A.
As shown in FIG. 14C, subsequently, developer such as alkali developer is used to eliminate the exposed portion 304 and make a recording master disk 306 including the master disk 301 and a concavo-convex pattern 305 formed with a photosensitive material on the master disk 301. Meanwhile, the exposed character information is formed as visible characters on the master disk 301.
As shown in FIG. 14D, subsequently, a layering technique for a thin film such as a sputtering or deposition method is used to form a conductive thin film 307 on a surface of the recording master disk 306.
As shown in FIG. 14E, subsequently, a metal plate 308 is formed by a method such as metal plating in which the conductive thin film 307 works as an electrode.
As shown in FIG. 14F, a laminate including the conductive thin film 307 and the metal plate 308 is peeled at an interface between the pattern 305 (photosensitive film 302) and the conductive thin film 307. The photosensitive material remaining on the surface of the conductive thin film 307 is removed with, for example, remover. Subsequently, the laminate is punch-molded into a disk, which is used as a metal stamper 309, so that the inner/outer diameter of the disk is fitted to a molding machine. The metal stamper 309 is thereafter used as a metal mold for molding a resin substrate.
Subsequently, the metal stamper 309 is used to mold a resin substrate by a resin molding method such as an injection molding process. Character information formed on an inner circumferential part of the metal stamper 309 is also transferred to the resin substrate. Typically, the metal stamper 309 is substantially consistently positioned with respect to the molding machine configured to mold the resin substrate. For example, the metal stamper 309 is mounted on the molding machine so that the character information formed on the metal stamper 309 is located at the top position. As a result, recording position of the character information always indicates the top direction of the molding machine. In the ensuing embodiments, a start position of the character information on the substrate is exemplified as the first reference point of the substrate.
A more moldable material such as polycarbonate is typically used as a resin substrate material. Subsequently, resin layers are laminated by a formation process of the resin layers such as a spin coating method as shown in Patent Document 1.
FIGS. 15A to 15I show a process for producing a dual-layered Blu-ray disc including processes for producing a resin intermediate layer (the resin layer 204) and the protective layer 207 according to a spin coating method. The process for producing a dual-layered Blu-ray disc is now described with reference to FIGS. 13 to 15.
Using the metal stamper 309 obtained through the processes described with reference to FIGS. 14A to 14F, the molded resin substrate 201 of approximately 1.1 mm in thickness is formed by a resin molding method such as an injection molding process. As described above, the concavo-convex first information surface 202 including pits and guide grooves is formed on one surface of the molded resin substrate 201. The first information layer 203 is formed on the first information surface 202 using a metal thin film or a thermally recordable thin film material by a sputtering or deposition method. As shown in FIG. 15A, the molded resin substrate 201 formed with the first information layer 203 is fixed on a rotatable stage 403 by a vacuum contact method and alike.
As shown in FIG. 15B, radiation curable resin A 404 is concentrically applied by a dispenser on an intended radius of the first information layer 203 of the molded resin substrate 201 fixed to the rotatable stage 403.
As shown in FIG. 15C, the rotatable stage 403 spins so that the radiation curable resin A 404 on the first information layer 203 is stretched and becomes a resin layer 406. The thickness of the resin layer 406 is controlled to become an intended value by adjustment of various parameters such as viscosity of the radiation curable resin A 404, spin speed, spin time, ambient atmosphere (for example, temperature and humidity) around the radiation curable resin A 404 during the spin, and so on. After stopping the spin, radiation is irradiated on the resin layer 406 from an irradiating apparatus 405 to cure the resin layer 406.
A transfer stamper 407 configured to form a second information surface 205 is prepared. The transfer stamper 407 may be molded, for example, by an injection molding process using the metal stamper obtained through the same processes as the series of the processes for producing the metal stamper 309 for use in molding the molded resin substrate 201 described with reference to FIG. 14F. Like the molded resin substrate 201, character information is also recorded at an inner circumferential part of the transfer stamper 407. In the embodiments described below, recording start position of the character information of the transfer stamper 407 is exemplified as the second reference point of the transfer stamper.
As shown in FIG. 15D, the transfer stamper 407 is fixed onto the rotatable stage 408 by a vacuum contact method and alike. Radiation curable resin B 409 is concentrically applied on an intended radius of the transfer stamper 407 on the rotatable stage 408 by a dispenser.
As shown in FIG. 15E, the rotatable stage 408 spins so that the radiation curable resin B 409 on the transfer stamper 407 is stretched to form a resin layer 411. Thickness of the resin layer 411 is controlled to become an intended dimension by controlling various parameters, similarly to the thickness control of the resin layer 406 described above. After stopping the spin of the rotatable stage 408, radiation is irradiated on the resin layer 411 from an irradiating apparatus 410 to cure the resin layer 411.
As shown in FIG. 15F, the molded resin substrate 201 formed on the resin layer 406 is fixed on the rotatable stage 413. Then, the transfer stamper 407 on which the resin layer 411 is formed is superposed on the molded resin substrate 201 on a rotatable stage 413 with interposing the radiation curable resin C 412. It should be noted that the transfer stamper 407 is integrated with the molded resin substrate 201 so that the resin layer 411 faces the resin layer 406.
As shown in FIG. 15G, the rotatable stage 413 supporting the integrated object of the molded resin substrate 201 and transfer stamper 407 spins so that the radiation curable resin C 412 is stretched under the control for an intended thickness of a resin layer 414 to be formed. Subsequently, an irradiating apparatus 415 irradiates radiation to cure the resin layer 414, so that the transfer stamper 407 is affixed to the molded resin substrate 201.
As shown in FIG. 15H, after the integration of the molded resin substrate 201 and the transfer stamper 407 by curing the layer of the radiation curable resin C 412, the transfer stamper 407 is peeled at the interface between the transfer stamper 407 and the resin layer 411, so that the second information surface 205 is formed on the molded resin substrate 201. The laminate of the resin layers 411, 414, 406 corresponds to the resin layer 204 described with reference to FIG. 13.
As shown in FIG. 15I, the second information layer 206 is formed on the second information surface 205 using a metal thin film or a thermally recordable thin film material according to a layering technique for a thin film such as a sputtering or deposition method. Subsequently, radiation curable resin D is applied on the second information layer 206 and subjected to a spin coating method, similarly to the formation methods of the resin layers 406, 411, 414. After the radiation curable resin D is stretched, radiation is irradiated to form cured protective layer 207. Optionally, a hard coat layer may be formed on the protective layer 207 to prevent defects such as scratches and fingerprints on the surface of the protective layer 207. A dual-layered Blu-ray disc is thereby completed. It should be noted that the radiation curable resin A 404 described with reference to FIGS. 15A to 151 preferably possesses favorable adhesiveness with the first information layer 203 and/or the resin layer 414. Meanwhile, preferably the radiation curable resin B 409 is more exfoliative from the transfer stamper 407 and possesses favorable adhesiveness with the resin layer 414. In addition, the radiation curable resin A 404, the radiation curable resin B 409, the radiation curable resin C 412 and the radiation curable resin D are substantially transparent to wavelength of the recording/replay light. In the processes described with reference to FIGS. 15A to 151, three types of radiation curable resins are used to form the resin intermediate layer (resin layer 204). However, for example, by appropriately selecting a material of the transfer stamper 407, fewer types of radiation curable resin may be used to appropriately control the peel-off of the transfer stamper 407 from the resin layer 204. Such a simplified method may also be suitably applied to the embodiments described below.
Patent Document 2 proposes a four-layered structured information recording medium including four information layers. Each of the resin intermediate layers in the four-layered structure information recording medium is different in thickness in order to moderate interference with the other layers. The thickness of the resin intermediate layer formed by a spin coating method is controlled to become an intended value by adjustment of various parameters such as viscosity of radiation curable resin, spin speed, spin time, ambient atmosphere (for example, temperature and humidity) around the radiation curable resin during the spin, and so on. In conventional technologies, a spin coating method is generally used for forming resin layers different in thickness like the four-layered structure information recording medium.
Higher capacity of Blu-ray discs is also demanded. For example, media including three information layers to achieve a capacity of 100 GB and media including four information layers to achieve a capacity of 128 GB have been proposed.
Multi-layering of two or more layers is achieved by repeating, several times, the process of forming the intermediate layer (resin layer 411) to the process of forming the second information layer 206 described with reference to FIGS. 15A to 15I. Several information layers are sequentially laminated as a result of repeating a series of the foregoing processes.
FIG. 16 is a schematic cross sectional view of a multi-layered structure of a Blu-ray disc medium obtained by repeating, several times, the process of forming the intermediate layer (resin layer 411) to the process of forming the second information layer 206 described with reference to FIGS. 15A to 15I. Drawbacks of the multi-layered structure medium are now described with reference to FIG. 16.
A medium 500 shown in FIG. 16 includes a substrate 501 of approximately 1.1 mm in thickness, and several information layers 502, 503, 504, 505 laminated on the substrate 501. In FIG. 16, a reference numeral “502” represents a first information layer which is the closest to the substrate 501. A reference numeral “503” represents a second information layer laminated on the first information layer 502 with interposing a resin layer. A reference numeral “504” represents a third information layer laminated on the second information layer 503 with interposing a resin layer. A reference numeral “505” represents an Nth information layer as the Nth layer far from the substrate 501. The first, second, third and Nth information layers 502, 503, 504, 505 are sequentially laminated toward the incident surface of recording/reply light 506. The medium 500 additionally comprises a protective layer 507 which covers the Nth information layer. The surface of the protective layer 507 becomes the incident surface of the recording/reply light 506.
All information layers 502, 503, 504, 505 in the multi-layered structure medium 500 are formed within a thickness dimension of approximately 0.1 mm from the surface of the protective layer 507 in order to moderate influence from tilt of the disc as described above. This means that, as shown in FIG. 16, the distance from the surface of the protective layer 507 to the farthest first information layer 502 from the protective layer 507 is limited to approximately 0.1 mm.
In order to achieve storage capacities demanded in three-layered media and four-layered media, higher storage capacity per information layer than conventional two-layered media has been proposed. A conventional two-layered medium has a capacity of 25 GB per information layer (that is, a total of 50 GB with two layers). For example, in order to achieve a capacity of 100 GB with a three-layered medium, a capacity of 33.4 GB is required per information layer. In addition, in order to achieve a capacity of 128 GB in a four-layered medium, a capacity of 32 GB is required per information layer.
In terms of compatibility of pickups and systems used in conventional BD media, it is undesirable to change guide grooves or pitches (track pitches). Accordingly, it is necessary to increase a line density in order to increase the capacity per information layer.
The increase in the line density results in shorter signal marks. As a result, deterioration in quality of the record/reply by a change in thickness of a protective layer and intermediate layers becomes more apparent than the density of 25 GB per information layer in a conventional two-layered medium. Accordingly, thickness dimensions from the surface of the protective layer to the respective information layers in multi-layered structure media have to be much more precise than a conventional medium with a density of 25 GB.
However, as a number of laminated resin layers increases from a conventional two-layered structure to three layers, four layers or more layers, variation in thickness from the surface of the protective layer to the respective information layers increases according to the process described with reference to FIGS. 15A to 15I.    Patent Document 1: Japanese Patent Application Laid-open No. 2002-092969    Patent Document 2: Japanese Patent Application Laid-open No. 2004-213720